1. Field of the Invention
The present invention relates generally to display panels and more specifically to an electroluminescent panel and a method of producing same, which exhibits a high yield and which enables the production of panels which are free from peel-off phenomenon and the like.
2. Description of the Prior Art
Japanese Patent Application first provisional publication No. Sho 54-155789 discloses a polychromatic electroluminescent element which is produced by forming on a transparent substrate, a plurality of layers (one directly on top of the other) which each produce different colours when excited.
However, this arrangement suffers from the drawback that it is possible only to generate a "mixed" colour (Viz., a colour which results from mixing the colours produced by each of the individual layers) as the layers cannot be individually excited.
Another previously proposed method of forming an electroluminescent panel, disclosed on pages 128 and 129 of the Syposium Digest 1982 of SID (Society of Information Display), allows individually excitement. This technique includes forming on a glass substrate 1, via vacuum evaporation deposition, sequential layers (having a thickness of 5000 .ANG. to 10,000 .ANG. in thickness) comprising (in order) a transparent electrode 2, a transparent insulating film 3 (e.g. Y.sub.2 O.sub.3), a layer 4 of ZnS:Mn (i.e. zinc sulphide doped with an impurity level of manganese), a second transparent layer 5 (e.g. Y.sub.2 O.sub.3), a second transparent electrode 6 having shape and orientation selected with respect to that of the first electrode 2, another transparent insulating film 7 followed by a layer 8 of ZnS:TbF.sub.3 (zinc sulphide doped with impurity levels of TbF.sub.3), and a final transparent insulating film 9 (for example Y.sub.2 O.sub.3).
Aluminium electrodes 10 and 11 are attached to the film electrode 6 and the insulating film 9, as shown.
With the above described construction, when an AC power of 1 KHz 200 Vrms is applied across electrodes 2 and 6 by a power source 12, the layer of ZnS:Mn becomes excited and emits orange light as indicated by arrow "O" while when a similar current is applied across electrodes 6 and 11 by power source 13, the layer of ZnS:TbF.sub.3 becomes excited and emits green light which is transmitted through the various layers as shown by arrow "G".
Although it would appear from FIG. 1 that the two beams of light "O" and "G" originate in different zones, they are in fact so close together as to effectively appear in a common zone.
Further, with the just described arrangement, it is possible to produce only two colours. Accordingly, to produce more colours it is necessary to add further layers including for example, ZnS:PrF.sub.3 which generates white light when excited, ZnS:DyF.sub.3 which generates yellow light, or ZnS:TmF.sub.3 which produces blue light.
However, where the number of colours to be produced is N, the number of layers which must be formed one on top of the other is 4N+1. Viz., for each colour five layers must be formed. This of course is a considerable number of layers and renders the arrangement susceptible to the so called "peel-off" phenomenon.
Experiments have shown that the sites where peel-off is most likely to occur is between the ZnS layers and the insulating film formed over same and between the intermediate electrodes and the insulating film on the electrode.
Moreover, with the above type of arrangement the transparent intermediate electrodes are provided by using photolithic techniques which require acidic etchants. For example, when iTo (a mixture of indium oxide and tin oxide) layers are used, the etchant takes the form of a 1:1:0.1 solution of H.sub.2 O, HCl and HNO.sub.3. Accordingly, during production, the ZnS layers which have little resistance to acidic solutions are often attached by etchant which leaks through the inevitable pin holes and cracks which occur from place to place in the insulating films.
It should be noted that although the intermediate electrodes can be formed using an evaporative masking technique, it is not possible to acheive the desired level of accuracy (i.e. pattern definition) when this technique is used.
The incidence of the above mentioned problems increase exponentially with N and thus renders the production of such units which produce more than 2 or 3 colours commercially impractical.